The present invention relates to an optical recording and reproducing apparatus for recording and reproducing information by using a plurality of light spots, and more particularly to a multi-spot positioning controller for causing a plurality of light spots to follow target tracks of a disk.
An optical recording and reproducing apparatus having a plurality of light sources has the following two problems to be resolved.
The first problem is to position to desired tracks two light spots formed by light beams emitted from two light sources and focused onto a disk through a common optical system. A track pitch is usually in the order of 1 .mu.m, and in order to read information on the target track with a satisfactory S/N ratio, a center of the light spot and a center of the track must be aligned at a precision of at least 0.1 .mu.m. Accordingly, in the prior art apparatus, where two separate light sources are used, it is necessary to position the light spot of each light source to the target track with the above precision.
When a monolythic array semiconductor laser having an activation layer on a substrate or a hybrid array semiconductor laser having a plurality of semiconductor laser chips on one mount is used, in order to form a plurality of light spots on a disk by light beams emitted from the respective light sources and passed through a common optical system, the array semiconductor laser should be coupled to the optical system such that the plurality of light spots are simultaneously positioned to their respective target tracks.
When information is recorded and reproduced to and from the disk by using a plurality of light spots, it is necessary to vary an angle between a line connecting two of the plurality of spots and a tangential direction of the disk. In JP-A-No. 61-214240, when a laser array having a plurality of light sources in one laser system (mount) is used as a multiple light source, an inclination of an optical head is changed by a lead screw or the laser system is inclined by an electrostrictive element. Where a plurality of independent lasers are used and the beams emitted from those lasers are focused by a common focusing optical system to form a plurality of spots on the disk, the inclination of the optical head is varied, the positions of the lasers are displaced perpendicularly to the optical axis, or an inclination of an optical element of an optical system for directing the beams of the lasers to the common focusing optical system is varied as shown in JP-A-No. 61-5443. When the positions of the plurality of light spots slightly vary by a temperature change, tracking error signals for the respective light spots are detected to change the above angles. In a tracking control system disclosed in JP-A-No. 61-5443 in which first and second light spots follow target tracks through a common focusing optical system, tracking error signals for the first and second light spots are detected to cause the light spots to follow the target tracks. In starting the control system, a first control loop is closed to cause the first light spot to precisely follow the target track to compensate for eccentricity and cause the second light spot to be roughly positioned to the target track. Under this condition, the second control loop is closed to compensate for a slight tracking error of the second light spot. Because of such a control system, a control gain of the second control loop may be much smaller than that of the first control loop, and a control band may also be narrow.
In the prior art apparatus, it has not been considered to examine whether the plurality of light spots were positioned to their respective target tracks and variably set the angle in accordance with the result of examination. As a result, even if the plurality of light spots are set in the optical adjustment stage such that they are positioned to the target tracks, the light spots may be positioned to other tracks than the target tracks by a temperature drift of the optical system, and this cannot be detected or compensated. In an apparatus having two recording and reproducing functions, for example, an apparatus having a function of rendering two light spots to follow one track with one light spot recording information while the other light spot reproducing the information, or a function for positioning two light spots to different tracks and record and reproduce information by the respective light spots, it is difficult to switch the functions. When disks of different nominal track pitches are used, the light tracks may be positioned to other track than the target track.
The second problem is to separate reflected light beams from the disk through the common optical system for the two light spots closely located on the disk and read information of the respective tracks. The separation method is classified into two major methods. One is a wavelength separation method which uses light sources of different wavelengths, and the other is a space separation method which spatially separates two slightly deviated reflected light beams.
One example of the wavelength separation method is disclosed in JP-A-No. 61-20235, in which two separate light sources having different wavelengths are used and the reflected light beams are separated by a dielectric mirror.
In this wavelength separation method, chromatic aberration due to wavelength scattering occurs by an element of the optical system which focuses the beams emitted from the two light sources onto the disk through a common optical axis. Accordingly, an optical element to cancel the chromatic aberration is added or an expensive dielectric mirror wavelength separation filter having a high separation resolution is used to reduce a wavelength difference of the light sources such that the influence of chromatic aberration can be neglected. With an element having two activation layers on one substrate such as a monolythic array semiconductor laser, it is difficult to produce light beams of different wavelengths.
An example of the space separation method is disclosed in '85 Optical Memory Symposium, pages 107-112, 1985, in which a reflected light beam from the disk is enlarged and focused, a reflection plate having a pinhole is located at a focus plane position with an angle to the focus plane, the reflected light beam for one light spot passes through the pinhole and the reflected light beam for the other light spot is reflected. Another example of the space separation method is disclosed in Applied Physics, 1986 Fall, 30p-ZE-2, in which the fact that beams emitted by a two-array laser have a slight angle difference on a common optical axis and the two reflected light beams having the slight angle difference are separated by a critical angle prism which is arranged such that an incident angle for one reflected light beam is larger than the critical angle and an incident angle for the other reflected light beam is smaller than the critical angle. When the space separation method is used, the problems encountered in the wavelength separation method do not arise, but an optical system for controlling automatic focusing and tracking for one of the separated light beams should be provided. Accordingly, a compact optical system cannot be provided.